1. Field of the Invention
Embodiments of the invention relate to a backlight unit capable of reducing a vibration of components affected by a vibration of lamps and a noise resulting from the vibration of the components and a liquid crystal display using the same.
2. Discussion of the Related Art
An application range of liquid crystal displays (LCDs) has showed a tendency to gradually increase because of characteristics such as thin profile, lightness in weight, and low power consumption of the liquid crystal displays. According to the tendency, the liquid crystal displays have been used in office automation equipment, audio/video equipment, and the like. The liquid crystal displays display a desired image on the screen by controlling an amount of light depending on a video signal applied to a plurality of control switches arranged in a matrix format.
Because the liquid crystal display is not a self-emission display device, the liquid crystal display needs a light source such as a backlight unit. The backlight unit is mainly classified into a direct type backlight unit and an edge type backlight unit. In the edge type backlight unit, a light source is positioned outside a liquid crystal display panel, and light from the light source is incident on the entire surface of the liquid crystal display panel using a transparent light guide plate. In the direct type backlight unit, a light source is positioned in the rear of a liquid crystal display panel, and light from the light source is directly incident on the entire surface of the liquid crystal display panel. In the direct type backlight unit, a luminance can be improved and an emitting surface can be increased because a plurality of light sources are used. Accordingly, the direct type backlight unit is used in a LCD TV needing a large-sized liquid crystal display panel.
A related art direct type backlight unit includes a plurality of lamps irradiating light onto a liquid crystal display panel, a lamp guide supporting the lamps, a bottom cover receiving the lamps and the lamp guide, a diffusion plate that is positioned on the lamps to diffuse light coming from the lamps, and a plurality of optical sheets that are stacked on the diffusion plate and change a traveling path of light from the diffusion plate in a direction vertical to the liquid crystal display panel.
The lamp guide includes lamp holders and a fame member. The lamp holders are supported by the frame member fastened to the bottom cover and fix the lamps on the bottom cover. A diffusion plate supporter is fastened to the fame member and prevents the diffusion plate from hanging down by heat of the lamps. A reflective sheet is attached to an inner wall surface of the bottom cover and reflects light travelling from the lamps to the bottom cover on the liquid crystal display panel. Hence, an efficiency of light incident on the liquid crystal display panel increases.
The lamps use a cold cathode fluorescent lamp (CCFL) or an external electrode fluorescent lamp (EEFL). In particular, a high-high manner, in which the lamps are parallel-driven by applying a high voltage AC signal to both sides of each of the lamps, has been recently proposed so as to reduce a volume of a large-sized LCD TV occupied by a backlight unit. In the high-high manner, balance printed circuit boards (PCBs) are respectively positioned at both sides of the lamps, and ends of the lamps are commonly connected to the balance PCBs. The high voltage AC signal is generated by an inverter PCB and then is supplied to the lamps through the balance PCBs. An emitting luminance of the lamps is determined by the inverter output signal of which a duty ratio varies by a burst dimming signal from a dimming circuit.
However, in case the lamps operate based on the inverter output signal, rapid energy changes are periodically generated in electrodes at both sides of the lamps due to the intermittently applied high voltage AC signal. As a result, the lamps slightly vibrate. The slight vibration of the lamps affects the balance PCBs commonly connected to the electrodes of the lamps. In other words, when a vibration frequency of the lamps is equal to a natural vibration frequency of the balance PCBs, the balance PCBs are resonated to generate a loud noise. The noise occurs louder in a front surface of the liquid crystal display than a back surface of the liquid crystal display completely covered by a back cover. In particular, the noise resulting from the vibration has gradually increased because of an increase in the number of lamps according to large size and thin profile of the liquid crystal display, an increase in a contact area between the electrodes and a structure of the lamps, and an increases in the inverter output signal for parallel-driving the lamps.
A transformer and a plurality of passive elements are mounted on the inverter PCB, and the transformer and the passive elements are protected from an external environment by fastening a shield cover shielding the inverter PCB to the inverter PCB. The shield cover is aligned through a plurality of L-shaped flanges guided by projections projecting from a back surface of the bottom cover and then is fixed on the back surface of the bottom cover by a screw coupling. However, because the flange is used to align the shield cover, bending angle of the flange is same before and after the shield cover is fastened to the bottom cover. Therefore, the flange is not firmly attached to the back surface of the bottom cover when the shield cover is fastened to the bottom cover. The bending shape of the flange is easily affected by the vibration of the lamps and thus causes a vibration of the shield cover. Hence, a noise results from the vibration of the shield cover.